His power derives from his immense vault of knowledge about seemingly every person on Earth's weaknesses – their "trigger points," he calls them. From the viewers' perspective, he accesses all that information via specially designed glasses that project data onto the lenses such that only he can see it. Are we supposed to be afraid of Google Glass now? But there's something fishy about those vaults at Appledore from the get-go…

The Mind Palaces

Moffat and co. have been hiding tiny glimpses and reminders of Sherlock's "mind palace" – his extensive means of organizing the hard drive of his brain – throughout the season, and last night's episode drew heavily on the concept, but not just for our "dragon-slaying" hero. Yes, it turns out Appledore is all in Magnussen's head as well.

To be fair, the first shots we get of his alleged basement vaults should've tipped us off; the old-school, scattered library feel just didn't fit in with the rest of his exceptionally modern home. But at the reveal in the third act, I couldn't help but think, "Lamest villain ever?" Lamest villain ever, though he cops out of that title by saying "I'm not a villain; I'm a businessman." He's creepy, sure, and it's certainly possibly to wreak plenty of havoc on people's lives with the right information in just the wrong hands, but let's be honest: Being a rich newspaper magnate in 2014 just doesn't have the same threatening feel to it as some kid on the other end of a keyboard halfway across the world.

The … Other Villain

We meet several residents of Sherlock's mind palace as he tries to sift through the best course of action after being shot in the gut by Mary – more on her in a moment – including Molly, Mycroft, and Anderson, but perhaps the biggest revelation was Moriarty, bound up in a straightjacket in a padded cell in the palace's basement. It's Andrew Scott in his most psychotic and (literally) haunting performance to date.

… That is, until we apparently get to see him again in season four?!

For an episode chock-full of gotchas and big reveals, the biggest by far was the pre-credits fake-out that brought Sherlock back (after all of four minutes) from the brink of an assignment in Eastern Europe that would almost certainly result in his death: Moriarty's return. Sure, it's possible it was pre-recorded or distributed by some other baddie with mischief on the mind, but the country-wide CCTV broadcast of that maniacal taunt is hard to deny.

Then again, so is the fact that we all watched Moriarty blow his brains out on the roof of the hospital in "The Reichenbach Fall." So what do we know.

Mary

Close second on the list of shocking reveals in this episode was Mary Watson's role as Magnussen's would-be assassin and, subsequently, her previous life. Over the course of a series of flashbacks-and-forths between actual time and Christmas a few months out, we realize the John is going to roll over and accept his wife for who she is, despite not actually knowing the answer to that question. It's a pitiable move given the hardened snark we saw out of his character in the earlier scenes of the episode, but one that will no doubt yield plenty of fodder for future fanfiction.

Plus, now you have an excuse to go back and watch the first two episodes of the season with her duplicity in mind.

Human Error

Human error was a common theme in this week's episode – not all of it intentional. Sherlock relies on it for would-be fiancée Janine to let him up to Magnussen's office, but what about the rest of these? The writers didn't explain how Mary got into the supposedly unenterable office. Mary didn't murder CAM when she had the chance, nor did she recognize John as the dummy at the end of the hall in Leister Square, thus revealing her secret to John. Sherlock's family fell for the drugged tea and punch, which isn't too uncharacteristic, but Sherlock doubted his instincts about Magnussen's spectacles, which decidedly was.

In fact, the one tidbit that originally looked like it might have been human error – Mary's not killing Sherlock at point-blank range – was eventually revealed by her skillshot of the coin in the hallway to be completely intentional.

Odds and Ends

• Ultimately, the episode title gave away the ending, in a way. "His Last Vow" isn't too hard to decode when just in the previous episode, Sherlock said:

"I've never made a vow in my life and after tonight I never will again, so here in front of you all, my first and last vow: Mary and John, whatever it takes, whatever happens, from now on I swear I will always be there, always, for all three of you."

• Other notable quotables:

"You know what happened to the other one." – Mycroft, implying a third Holmes brother

"Sherlock Holmes, if you've been YouTubing…" – Mrs. Hudson, trying to deny her past as an exotic dancer

"Oh you're not getting better, are you?" – Moriarty to Sherlock as he fights back from flatlining, in possibly one of the most ironic quotes to date

"Don't reply; just look frightened and scuttle." – Mycroft to the members of the Empty Hearse after Sherlock spoke Magnussen's name in front of them

• Two key souvenirs from Sherlock's time at the crackhouse: "protege" Bill Wiggins and those priceless attempts by costuming to make Benedict Cumberbatch unattractive by putting a little dirt on his face and the world's worst-tailored clothes on his back.

• And what of a fourth season? "Frankly, starvation works," Moffat told Empire Online after the finale aired across the pond. All the cast and crew are on board to continue doing this fun little project whenever schedules align, and Moffat and Mark Gatiss already have the next two series mapped out – lord knows they left themselves plenty of questions to answer – but don't hold your breath for those to make their way to your screens anytime soon.